The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Referring now to FIGS. 1-6B, wireless network devices typically transmit and receive radio frequency (RF) signals via RF transceivers. FIG. 1 shows an exemplary RF transceiver. FIG. 2 shows exemplary RF signals including wanted and unwanted RF signals that an RF transceiver may receive via a source such as an antenna. FIGS. 3A and 3B show different exemplary configurations of RF transceivers that can simultaneously transmit and receive RF signals. FIG. 3C shows a duplexer that may be a source of unwanted RF signals. FIGS. 4-6B show different exemplary filter configurations that may be used to filter some of the unwanted RF signals.
In FIG. 1, an RF transceiver 100 that transmits and receives RF signals in wireless communication systems is shown. The RF transceiver 100 may transmit and receive RF signals in a desired frequency band. The RF transceiver 100 may comprise a transmitter 102 that transmits RF signals and a receiver 104 that receives RF signals via an antenna 106. The transmitter 102 may be a super-heterodyne transmitter, a direct conversion transmitter, or other suitable transmitter. The receiver 104 may be a super-heterodyne receiver, a direct conversion receiver, or other suitable receiver. The RF transceiver 100 may be implemented by an integrated circuit (IC).
Transmitters and receivers generate unwanted signal components called intermodulation components due to non-linearity of circuit blocks. For example, receivers may generate intermodulation components due to non-linearity of low-noise amplifiers, transconductors, mixers, amplifiers, and filters. Mixers are a significant source of nonlinearity, particularly in direct conversion receivers.
When receivers receive a wanted channel at a small power level together with unwanted signals with relatively larger power, intermodulation components may fall on or near the wanted channel and reduce receiver performance. Receivers typically use mixers to convert input signals associated with one carrier frequency to output signals having another carrier frequency called an intermediate frequency (IF) or to baseband as in receivers employing direct conversion architecture. Generally, receivers may receive unwanted input signals having relatively large power from different sources.
In FIG. 2, the receiver 104 may receive signals having different frequencies and amplitudes via the antenna 106. For example, the receiver 104 may receive wanted signal 108 having frequencies in the desired frequency band. Additionally, the receiver 104 may receive unwanted or interfering signals called blockers. Blockers may be of two types: In-band blockers 110 that have frequencies in the desired frequency band and out-of-band blockers 112 that have frequencies outside the desired frequency band. The power of the blockers 110, 112 may be greater than the power of the wanted channel 108. Consequently, the blockers 110, 112 may generate unwanted intermodulation components when received by the receiver 104.
Additionally, signals transmitted by the transmitter 102 may be a source of blockers in some systems. For example, in wideband code division multiple access (WCDMA) systems, RF transceivers may comprise RF front-end modules that include duplexers. The RF transceivers may simultaneously transmit and receive data using the duplexers.
In FIG. 3A, a WCDMA transceiver 100-1 may comprise an RF front-end module 114, an RF downconverter module 116, and the transmitter 102. In some implementations, the RF front-end module 114 may comprise filter circuits and may be implemented external to an RF transceiver 100-2 as shown in FIG. 3B. The RF front-end module 114 may refer to components housed in a single enclosure or may be a functional grouping of such components.
In FIGS. 3A and 3B, the RF front-end module 114 may communicate with the antenna 106, the RF downconverter module 116, and the transmitter 102. The RF downconverter module 114 may include mixers (not shown) that downconvert the RF signals received from the antenna 106. The RF front-end module 114 may comprise filters, amplifiers, etc. that the receiver 104 and/or the transmitter 102 may utilize. Accordingly, the receiver 104 may include the RF downconverter module 116 and a portion of the RF front-end module 114 that the receiver 104 may utilize.
In FIG. 3C, the RF front-end module 114 may comprise a duplexer 120 and a power amplifier 122. The RF transceiver 100-2 may simultaneously transmit and receive data via the duplexer 120. The power amplifier 122 may amplify signals output by the transmitter 102. When the RF transceiver 100-2 transmits data, the duplexer 120 may output the amplified signals received from the power amplifier 122 to the antenna 106.
When the RF transceiver 100-2 receives data, the duplexer 120 may receive RF signals from the antenna 106 and may output the signals received from the antenna 106 to the RF downconverter module 116. Additionally, the duplexer 120 may inject residual signals, which are attenuated portions of the amplified signals output by the power amplifier 122, into the RF downconverter module 116.
Despite being attenuated, the residual signals may have a power level significantly greater than the power of the wanted channel received from the antenna 106. Consequently, the residual signals may appear as large blockers at the input of the RF downconverter module 116 and may result in the generation of unwanted intermodulation components in the RF downconverter module 116.
Unwanted intermodulation components may be minimized by using highly linear circuit blocks (i.e., circuits with high second-order input intercept point (IIP2) and third-order input intercept point (IIP3) ratings). Generally, the relationship between the power of blocker(s) to the power of the wanted channel and their relative frequencies determines the IIP2 and IIP3 ratings required for the circuit blocks. The greater the ratio of the power of the blocker(s) to the wanted channel, the higher the IIP2 and IIP3 ratings required for the circuit blocks to generate acceptably low-power unwanted intermodulation components.
In receivers using direct conversion architectures, mixers with high IIP2 ratings are typically required. The implementation of highly linear mixers may, however, be impractical due to design and cost constraints. Calibration methods may be utilized to achieve highly linear mixers. However, even after calibration is performed, intermittent calibration or background calibration may be required to track temperature and power supply variations. This can be problematic in some applications where continuous operation is required.
Instead, the unwanted intermodulation components may be minimized by attenuating blockers relative to the wanted channel using filters that precede the mixers so that mixers with relaxed IIP2 and IIP3 ratings may be used. Generally, the lower the power (i.e., the greater the attenuation) of the blocker(s), the lower the IIP2 and IIP3 ratings necessary for the mixers. For example, attenuating blockers before the input of a mixer by 1 dB may decrease the IIP2 rating of the mixer by 2 dB.
Blockers may be attenuated by using a variety of filters. Typically, RF transceivers implemented by integrated circuits (ICs) may have on-chip filters. The on-chip filters, however, may be unable to adequately filter blockers relative to the wanted channel to provide significant relaxation of the linearity requirements of the circuit blocks. As a result, additional filters may be arranged external to the ICs comprising the RF transceivers to attenuate blockers. For example, filters including surface acoustic wave (SAW) filters, film bulk acoustic resonator (FBAR) filters, and/or LC tank filters can be used preceding circuit blocks to attenuate blockers and relax linearity requirements for the circuit blocks that follow.
In FIGS. 4-6B, different exemplary filter configurations for attenuating blockers are shown. In FIG. 4, a SAW filter 126 is arranged externally to the IC comprising the RF transceiver 100-2 to attenuate blockers. An RF front-end module 114-1 may comprise a low-noise amplifier (LNA) 124 that communicates with the duplexer 120 and amplifies the signals received from the duplexer 120. The LNA 124 may amplify both the RF signals received by the duplexer 120 via the antenna 106 and the residual signals. The LNA 124 may output the amplified signals including amplified residual signals to the RF downconverter module 116. The SAW filter 126 may attenuate the amplified residual signals.
Alternatively, SAW filters and/or LC tank filters (e.g., band-pass or notch filters) may be included in RF front-end modules as shown in FIGS. 5 and 6A, respectively. In some implementations, the SAW filter 126 alone may precede the RF downconverter module 116 as shown in FIG. 6B.
In FIGS. 5-6B, the RF downconverter module 116 may comprise a downconversion mixer 105. The downconversion mixer 105 may include an LNA 128 and mixers 132 and 134. The mixers 132 and 134 may be arranged in a quadrature configuration. An oscillator 136 and a 90-degree phase shifter 138 may generate clock signals that clock the mixers 132 and 134, respectively. The mixers 132 and 134 may generate in-phase (I) and quadrature (Q) outputs, respectively. The I and Q outputs may be input to baseband processing circuits (not shown) for further processing.
In FIG. 5, an RF front-end module 114-2 may include the SAW filter 126. The SAW filter 126 may receive input from the LNA 124, may attenuate blockers, and may generate an output that is input to the LNA 128. The LNA 128 may amplify wanted signals that are input to the mixers 132 and 134. In FIG. 6A, an LC tank filter 130 may be included in an RF front-end module 114-3 to attenuate blockers. To substantially attenuate blockers, however, filters such as the LC tank filter 130 may need to have a very high Q-factor (e.g., Q of the order of 100 or greater).